Anti-mana was an I.C.E. Age concept reintroduced by the Vvrael Saga. It typically takes the form of tendrils or voids, due to the sentient nature of the Vvrael. While the Vvrael is "collective sentient anti-mana", this does not imply all anti-mana has this property, much as a deity could be thought of as "sentient mana" without all mana being conscious with independent will. Its practical effect is the draining of magical power from those who touch it, though it would seem to be a form of energy for making the undead. Anti-mana has been outside of the context of The Rift during mana storms and in situations of portals being opened to other valences. However, its intrinsic nature is not well-defined, nor its relationship with mana.
Behind the Scenes
In the Shadow World cosmology, Essaence was the foundation of all Existence, which ran between poles of Order and Chaos. Material "planes of existence" were lowly ordered, whereas the chaotic realms were the home of demons and the dark gods. At the very furthest extreme was The Unlife, the "heart of Chaos", which was pure Anti-Essaence and so inherently destructive of all existence. While there were also Outer Planes that were not part of Existence, such as the Pales and the Black Hel, the "near" ones only resembled our own due to accidental parallels with the axis of Chaos. Some Shadow World books treated The Void as an origin of Unlife, and others as being outside of that scheme entirely.
The Dark Gods were originally "manifestations of the chaotic aspects of the Anti-Essaence", prior to the introduction of the Arkati concept for the modern history. (The Light Gods had been stranded here from a higher plane of existence, as a result of a cataclysm which allowed magic to exist in our world, which were generally inaccessible from lower ordered realities.) They were not the furthest extreme of chaos or "negation" as were true servants of Unlife. With the introduction of "anti-mana" as a power for creating the undead, and the Dark Gods now being considered the same race and nature as the Orhan (Liabo) pantheon, the unholy nature of undeath and the precedent of extra-planar entities causing their existence was much more ambiguously defined. The Vvrael is a cross between the Agoth and Unlife concepts from Shadow World, essentially a collective extra-planar entity of pure "anti-mana", and so inherently destructive of all existence.
In the I.C.E. Age form of the concept, it was supposed to be so alien to our own existence, the most powerful mages would only be able to detect it as an absence in the background radiation that surrounded all things. Whatever dark magics used it would seem to be another source of power entirely. Dark Essaence was sometimes described as "a fragmentation of a normal flow", draining the energy of whatever it touched and forming a vortex around "some accursed place." It was not possible to wield the dark essence without corrupting oneself, literally a form of metaphysical radiation poisoning. The undead can be thought of as a debasement of order, and the demonic as violence against nature itself.